1. Field of the Invention
The present invention relates to a method for melting a fluorophosphate glass. The fluorophosphate glass obtained according to the method of the present invention can be used for a laser amplifier for laser nuclear fusion, a laser window, an optical lens, etc.
2. Description of Prior Art
Phosphate glasses and fluorophosphate glasses used as a laser glass are produced by melting in a platinum container. In this production, however, contamination of said glass by platinum inclusions caused by dissolution of container platinum into the glass has been inevitable. Platinum inclusions cause the thermal destruction of said glass by the laser beam and, accordingly, the acceptable upper limit of the platinum inclusions is becoming severer (lower) with the recent increase of laser output, that is, the limit is now controlled below a microscopically detectable level. Hence, the decrease or elimination of platinum inclusions in the melting of a phosphate glass or fluoprophosphate glass in a platinum container is an important task in production of laser glass.
It was attempted to melt said glass in a clay crucible or a silica or alumina crucible in place of a platinum container. In this attempt, however, the crucible materials were chemically attacked and a glass has not been obtained yet which has a high quality required for laser glasses and which is free from impurities and coloring.
Use of an amorphous carbon crucible was tried for melting a zirconium fluoride glass (the resulting glass is expected to be utilized as a material for low less optical communication fiber, and a relatively good quality glass was obtained on laboratory scale [Materials Science Forum, Vol. 5, pages 1 to 18 (1985), H. G. Drexhage]. However, the literature makes no mention of any oxide glass and oxide-containing glass.
Heretofore, it has generally been thought that a high quality glass can not be obtained by melting an oxide glass or oxide-containing glass in a carbon container, because the container carbon is oxidized to generate bubbles in the glass and further the oxide of the glass is reduced by the carbon.
No case has been reported yet in which any fluorophosphate glass was melted in a carbon container.
Hence, the object of the present invention is to provide a method for melting a fluorophosphate glass, which is particularly useful for obtaining a high quality fluorophosphate laser glass free from platinum inclusions or coloring.